The second Beatitude is more than it seems on the surface. It seems like Jesus is just saying not to worry, if you are sad about something God will comfort you. But Jesus is talking about being Blessed, about being Happy. How can those who are mourning be happy or feel blessed?
It is true that God will bring you comfort when you need it most. Jesus called the Holy Spirit "Comforter" for a good reason. Many and many are the times when people have turned in their grief to their Creator and found the reassurance they needed to get them through the darkest times in their lives. But this verse is talking about more than that kind of relief from sadness.
How can you be happy when you are sad? It depends what you are sad about. Jesus was concerned for our earthly lives, He shows that time and again in the Gospels. His compassion is our example of the way our Creator feels about us. But Jesus did not come just to help us through our earthly struggles and to help us to understand who God is. He also came to provide a way that we could be reconciled to God and have a Father/child relationship with Him. However, sin separates us from our loving creator and makes such a relationship impossible. Sin is something we can do nothing about by ourselves. It is our nature to sin, sometimes I think we sin as easily as we breathe. And that sin keeps us from what our souls crave, a relationship with our Creator. It makes an abyss that separates us.
Jesus was pointing out that those who come to a time in their lives when they realize this separation from God will mourn, they will cry and they will grieve. They will stand on their side of the great gulf that separates them from their Creator and they will long with tears for a way to bridge it. And when that moment comes, they will find that Jesus has made a way to bridge that gap for them and that He gave His own life to do it! What a great comfort it is to know that Christ did what we could not do for ourselves and that He did it willingly and lovingly. He stands on our side of the great abyss and offers His gift to us freely. All we have to do is believe and accept it. And our sadness for our sinful nature will be time and again comforted with the knowledge that this is the gift that truly keeps on giving. "Godly sorrow brings repentance that leads to salvation and leaves no regret" (2 Corinthians 7:10 NIV)
So true grief can lead to great happiness in our lives. It is a blessedness that will carry us through the dark times here on earth and keep us focused on our relationship with God. And that relationship is where our true happiness lies, it transcends all of our struggles and pain. It overcomes even death: as Isaiah says "He will swallow up death in victory; and the Lord GOD will wipe away tears from off all faces" (Isaiah 25:8 KJV). So in this Beatitude, Jesus brings us true Comfort. Not only earthly comfort, but eternal comfort as well.
"For his anger endureth but a moment; in his favour is life: weeping may endure for a night, but joy cometh in the morning" (Psalms 30:5 KJV)